This invention is concerned with the extrusion of sheets of plastics or other extrudable materials and in particular the invention relates to the manufacture by extrusion of non-uniform sheet materials, i.e., materials which are not of a constant thickness or are of heterogeneous composition, and which term is used herein to include lattice structures of connected filaments, perforated sheets, and sheets incorporating thickness variations or filaments for reinforcement. Furthermore, "sheet material" should be understood to include non-planar forms, such as cylinders, tubes and other sections besides generally flat layers.
In accordance with the invention there is provided an apparatus for extruding non-uniform sheet material comprising die means including a first part at least partly delimiting extrusion orifice means around an axis, and a second part arranged to cooperate with the first part at the orifice means, and drive means for imparting relative movement between the first and second die parts whereby the second part undergoes a rocking motion progressing along at least part of a generally circular path around the axis relative to the first part and thereby the second part influences the extrusion emanating from each section of the orifice means in a periodic manner.
In order to achieve the relative movement of the first and second parts of the die means, the first part, the second part, or both parts may be driven by the drive means.
The first and second parts may have surfaces which cooperate to define the orifice means. In this case the first and second parts provide inner and outer annular die surfaces, which may be arranged to extrude several continuous filaments spaced apart from each other, and to co-extrude by the relative rocking motion transverse filament sections interconnecting and spaced apart along the continuous filaments. The simultaneously extruded transverse and longitudinal continuous filaments can be substantially perpendicular to each other.
In another construction the first die part defines the extrusion orifice means and the second part controls the flow of material through the orifice means or acts on the material flowing out through the orifice means to vary the extrusion obtained from each element or section of the orifice means about the axis. Thus, the second part may, as a result of the rocking motion, open and close an extrusion slot extending about the axis so that flow through the slot is alternately allowed and interrupted and a transverse or helical filament is produced, or the second part may partially open and close an extrusion slot extending about the axis so that the thickness of the sheet extruded therethrough is varied. Alternatively, the second part may be adapted to produce holes in the extruded sheet by temporarily closing off segments of the extrusion orifice to block flow therethrough, or by stamping holes in the extruded sheet.
Moveable members for controlling or varying flow of molten material through annular extrusion slots have been known for many years, but these members are reciprocated, which is a disadvantage from the point of view of operating smoothness and possibly speed. It is also known to extrude filaments between die members which are rotated relative to each other, but the extrusion openings obtained exhibit lozenge-like shapes which can be undesirable. By employing a rocking motion, i.e., a combined pitching and rolling movement without relative rotation of the first and second parts, in accordance with the present invention, smooth continuous high speed operation is facilitated. The invention also offers greater versatility than the prior art, especially regarding the different forms of extrusion which can be manufactured. One novel form of extrusion which can be produced comprises a sheet of material with continuous longitudinal filaments on one side thereof and transverse filaments on the other side thereof, the filaments on at least one side being of a material, e.g. polypropylene, different to the sheet material, e.g. polyethylene.
The first and second die parts preferably have annular frusto-spherical surfaces in sliding contact for guiding the two parks for the relative movement which is about the geometric centre of the frusto-spherical surfaces, and for sealing the die parts to each other to prevent ingress of extrusion material between them. It is preferable that one die part remains stationary while the other die part is moved, and most conveniently the first die part, i.e. the inner part, is fixed and the second or outer part is driven with the circular rocking motion.
It will be appreciated that the apparatus of the invention is mainly aimed at producing an extrusion consisting of a generally annular curtain with means, e.g. a row of holes, a reinforcing filament or increase in thickness, or a slot, which extends helically about the curtain. However, it is not essential for the orifice means to extend around a complete annulus and a longitudinally slit tubular extrusion, or longitudinal extruded strips could be produced.
The present invention also resides in a method of producing an extruded sheet, comprising extruding material through an extrusion orifice means extending at least partly around an axis, and varying the form of the extrusion emanating from the orifice means by imparting relative movement between first and second die parts so that the second die part performs with respect to the first die part a motion progressing repeatedly along at least part of a generally circular path without rotation about the axis. Most conveniently the movement is a circular rocking motion.
In addition, the invention provides an extrusion of generally cylindrical configuration having at least one continuously longitudinally extruded element and a feature extending helically therealong and formed during the extrusion process.
The helical feature may be a helical filament or a reinforcement produced by an extruded filament or an increase in thickness. Alternatively, it may be a row of holes in the extrusion or even a continuous slit. The basic cylindrical extrusion may comprise a series of separate parallel elements or a continuous sheet.
The present invention will be of particular benefit in the production of extruded lattice structures having a plurality of parallel longitudinal filaments and at least one continuous extruded helical filament interconnected with the longitudinal filaments, such structures being suitable for subsequent stretching to form plastics nets, e.g., for agricultural and horticultural uses.